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a poorly metamorphosed, fine-grained carbonaceous or siliceous slate in the form of a black bar that is used to determine the fineness of a precious metal from the color of the streak made by the metal on the stone.
The basic requirements for a touchstone are a hardness of 4.7–6.5 on Mohs’ scale and an 8- to 23-percent C content; in addition, a touchstone can contain no more than 2 percent admixed materials, such as Al, Fe, Ca, Mn, S, Na, and Cl. It should have no fractures and should not react with inorganic acids or their mixtures.
In the USSR rocks called schungites, which meet the above requirements, are found in the Urals, at the mouth of the Chernaia Aragvi River near Tbilisi, and in the Usovaia Gora quarry in the Karelian ASSR. Rich deposits of siliceous slate are located in the western part of Asia Minor, in the region previously occupied by the ancient country of Lydia, from which derives the name Lydian stone for the rock.
A process has been developed for the manufacture of artificial touchstones by sintering or smelting a batch of a certain composition that has been prepared in advance. However, these stones are inferior in quality to high-grade natural touchstones.